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volved about twenty times per second, it is rendered very flat by centrifugal action. It can then be brought between points or balls, even when the latter are separated by no more than millim. When in this position, the discharge between the points or balls perforates the disk and leaves a permanent record of its character, of the duration of the whole discharge, and of the intervals separating its constituent flashes and sparks. To obtain the time of rotation of the disk, I use the method invented by Young in 1807 (see his Natural Philosophy,' vol. i. p. 191); that is, I present momentarily to the rotating disk a delicate point which is attached to a vibrating tuningfork. The number of vibrations per second of this fork has been determined to the last degree of precision by means of a breakcircuit clock, which sends at each second a spark from an inductorium through the fork's sinuous trace on blackened paper covering a revolving cylinder. The axis of the sinuous line on the disk is traced with a needle point; and then, on drawing radii through symmetrical intersections of this axis on the sinuous line, we divide the disk off into known fractions of time. The disk is now removed from the rotating apparatus, and the carbon is fixed by floating the disk for a moment on thin spiritvarnish. When the disk is dry and flat it is centred on a divided circle provided with a low-power reading-microscope; and the duration of the whole discharge, and the intervals separating its components, can be determined to the 5000 of a second.
Many results have been obtained with this apparatus. I defer their publication until I have carefully examined them and have extended this research with the study not only of the discharge of the inductorium, but also of the frictional machine, of the Leyden jar, and of the Holtz machine, under every condidion of charged surface and of striking-distance, and when the current is flowing freely over a conductor and when it is doing work. I here present, merely as examples of the value of the method, the results I have obtained in three conditions of experiment.
1. Discharge of large inductorium* between platinum points one millim. apart. No jar in the circuit.
The platinum electrodes were neatly rounded and formed on wire millim. in diameter. After the discharge through the rotating disk, nothing was visible on it except a short curve formed of minute, thickly set white dots; but on holding the disk between the eye and the light, it was found to be perforated with thirty-three clean round holes with the carbon undisThe striking-distance of this coil between brass points was
turbed around their edges. The portion of the discharge which makes these holes lasts second; and the holes are separated by intervals which gradually decrease in size toward the end of the discharge, so that the last spark-holes are separated about one half of the distance which separates the holes made at the beginning of the discharge. The average interval between the spark-holes is second. After this portion of the discharge has passed there is a period of quiescence lasting about T second; then follows a shower of minute sparks, which forms the short dotted line above spoken of. This spark-shower lasts 330 of a second, and is formed of 30 sparks; hence the average interval separating these sparks is second. The intervals separating these sparks, however, are not uniform, but are smaller in the middle of the spark-shower than at the beginning and at the end of this phenomenon. The spark-shower, indeed, is a miniature of the phenomenon obtained when a Leyden jar is placed in the circuit of the coil, and which is described below. The above numbers were determined as the average measures on six disks. It is here to be remarked that all the discharges studied in this paper were made by suddenly depressing the platinum-faced "break" of the primary circuit, and the break was held in this position until the disk had been removed from between the points or balls.
2. Discharge of large inductorium between platinum points one millim. apart, with a Leyden jar of 242 sq. centims. connected with the terminals of the secondary coil.
After this discharge through the disk a very remarkable appearance is presented, the full description of which I reserve for a more extended paper. The discharge in its path around the disk dissipates little circles of carbon. There are 91 of these circles, each perforated by 4, 3, 2, or 1 hole. I shall here have to adopt a new nomenclature for the description of this complex phenomenon. I call the whole act of discharge of the coil the discharge. Those separate actions which form the little circles by the dissipation of the carbon I denominate flashes; and the perforations in these circles I call sparks. The discharge in the above experiment lasts of a second. The flashes at the beginning of the discharge are separated by intervals averaging second up to about the tenth flash; after this the intervals of the flashes rapidly close up, so that during the fourth fifth of the discharge they follow at each of a second. During the last fifth of the discharge the intervals. between the flashes gradually increase, and the last flash is separated from its predecessor by Too of a second.
3. Discharge of large inductorium between brass balls, one centim. in diameter, separated one millim., with a Leyden jar of Phil. Mag. S. 4. Vol. 49. No. 322. Jan. 1875.
242 sq. centims. inner coating, connected with the terminals of the secondary coil.
This discharge also lasts second, and is similar to the preceding, except that larger circles are made on the disk by the dissipation of the carbon, and that there are fewer flashes, viz. 71. The total number of spark-holes in these flashes is 123. Thus there are fewer flashes than in the experiments with the platinum points, but the total number of spark-holes is the same in each case. Hence there is, on an average, 1.34 spark to each flash with the points, and 1.73 spark-hole to each flash with the balls.
Experiments have also been made with rotating disks formed of "sensitized " paper; and interesting results have been
VI. On Polarization by Diffusion of Light.
On the Reflecting-power of Flames.
PUBLISHED, some months since, a first Note†, on the occasion of a memoir by M. G.-A. Hirn, in which he put forth the hypothesis that the incandescent solid particles which, according to Davy's theory, produce the brightness of flames, become transparent at the high temperature to which they are raised, and no longer possess any sensible reflecting-power. One of the arguments which he advances in support of his hypothesis consists in the fact that no phenomena of polarization are observed in the light of a flame exposed to the rays of the sun.
I indicated the results I had obtained by causing a pencil of solar light to fall on lampblack, either when deposited on another body, or at the moment of its formation-that is to say, when it is in the state of smoke or of a smoky flame. In this latter case the trace of the pencil of solar rays is perfectly visible: the part of the flame which receives the rays appears bluish white, contrasting with the reddish tint of the adjacent parts. If the trace be observed with an analyzer, the light diffused in a direction at right angles to the incident pencil is seen to be completely polarized in the
* Translated from a separate impression, communicated by the Author, from the Archives des Sciences of the Bibliothèque Universelle, July 1874, pp. 1-26. An abstract of this memoir has been communicated to the Paris Academy.
"On some Phenomena of Polarization by Diffusion of Light" (Archives, Nov. 18, 1873, vol. xlviii. p. 231), Phil. Mag. S. 4. vol. xlvii. p. 205. I take this opportunity to point out two important misprints in that Note (Archives, pp. 235, 238): Phil. Mag. t. c. p. 208, line 12-13, for horizontal read vertical; p. 209, line 10 from bottom, for 50° read 90°.
plané of vision, so that the white trace ceases to be visible when the analyzer is turned so as to intercept the rays which are polarized in the plane containing the eye and the pencil of solar light.
When the flame is not smoky and when complete combustion augments its brilliance, I had found, like M. Hirn, that the polarization-phenomena are not sensible; but I expressed some doubts of the necessity of concluding from this the absence of the reflecting-power of the particles at a high temperature; the dazzling of the eye and the less quantity of these particles in a brilliant flame, in comparison with a smoky.flame, appeared to me sufficient to account for the facts.
I have since endeavoured to control this way of viewing the subject by concentrating the sunlight much more than I had previously done, so as to give more brightness to its trace. The sunlight, reflected from a silvered mirror, falls upon a good achromatic lens of 72 millims. aperture and 1.5 metre focal length. Lastly, when greater concentration is required, a second lens, much more convergent, is added, near the focus of the first. The flame is then placed at the point where the image of the sun is found.
Working thus with different flames proceeding from carburetted substances, the trace of the sun's rays can be perceived, in most cases, very distinctly, and the usual phenomena of polarization ascertained. When the flame is not too bright, and does not fatigue the eye, the observation is readily made, by aid of a Nicol, with the naked eye; but if the flame is dazzling, there is a great advantage in looking through one or several plates of blue (cobalt) glass. The portions of the flame which do not receive the light of the sun appear then of a purple tint, while the trace of the pencil is clearly distinguished by its blue colour. If observed through the Nicol in the proper position, the blue trace disappears, and the whole flame appears purple.
I have verified these facts in the following cases :—the flame of a wax candle; flame of ordinary gas from a Bengel burner with a glass chimney, or from a butterfly burner; flame of illuminating gas strongly carburetted, butterfly burner; flame of a petroleum lamp, and of a moderator lamp with oil.
I finally tried the very brilliant flame obtained by burning illuminating gas strongly carburetted, with the addition of oxygen. With the process of concentration of the sunlight above described, the trace and its polarization are still distinctly observed as long as the oxygen is not too abundant, the brightness being, however, already incomparably more vivid than that of an ordinary flame.
By employing more energetic means of concentration that
is to say, by causing the sunlight, reflected by the large mirror of a siderostat, to pass through an objective of 8 inches aperture, and then through a lens of short focus, I could perceive the trace on this flame of carburetted gas fed by a larger proportion of oxygen than in the preceding case; but when the oxygen became too abundant, the trace was no longer visible— which, beside difficulties of observation, may be explained, 1st, by the fact that, the flame having become quite white and even bluish, there is no longer any difference of tint between the part which receives the solar rays and the parts which do not receive them, the trace can only manifest itself by a difference of intensity more difficult to perceive; 2nd, by the carbon particles being immediately consumed at the moment of their formation, and consequently the reflecting matter becoming relatively much rarer.
In short, these experiments show that carbon retains its reflecting-power at very elevated temperatures, which temperatures it would nevertheless be difficult to state precisely.
Further, these facts appear to me to have some interest because they confirm, at least for ordinary flames, the theory of Davy, which has recently been strongly contested; in fact, a pencil of solar light is reflected by diffusion and polarized in precisely the same manner, whether it falls on a very brilliant flame, or whether it illuminates non-incandescent smoke, in which the presence of carbon particles is incontestable.
The Cause of the Illumination of Transparent Bodies and
In my former publications I have maintained the opinion that the illumination of transparent bodies traversed by a pencil of rays must be attributed to a defect of homogeneity in the medium-a defect consisting most frequently in the dissemination of foreign particles of great tenuity, but may also result from differences of refrangibility in the component parts of the medium, or, in the case of a solid body, from minute voids or fissures. In other terms, illumination is for me only a particular case of diffusion of light.
My learned friend M. Lallemand attributes this phenomenon to the molecules themselves of the transparent body; he regards the illumination as a lateral propagation of the incident luminous motion, caused by the condensation of the æther around each molecule. Thus, for him, a pencil of light traversing a body which is transparent, not fluorescent, and absolutely homogeneous, must in general give rise to a trace visible laterally, and the phenomenon must depend essentially on the nature itself of the medium in which it is produced.